The Americas, the last continents to be entered by modern humans, were colonized during the late Pleistocene via a land bridge across what is now the Bering strait. However, the timing and nature of the initial colonization events remain contentious. The Asian origin of the earliest Americans has been amply established by numerous classical marker studies of the mid-twentieth century.

More recently, mtDNA sequences, Y-chromosome and autosomal marker studies have provided a higher level of resolution in confirming the Asian origin of indigenous Americans and provided more precise time estimates for the emergence of Native Americans. But these data raise many additional questions regarding source populations, number and size of colonizing groups and the points of entry to the Americas.

Rapidly accumulating molecular data from populations throughout the Americas, increased use of demographic models to test alternative colonization scenarios, and evaluation of the concordance of archaeological, paleoenvironmental and genetic data provide optimism for a fuller understanding of the initial colonization of the Americas.

In the figure, it can see the hypothesized routes for original migration into the Americas.The Beringian and Pacific coastal routes (blue and yellow, respectively) may have been roughly contemporaneous following the Last Glacial Maximum (LGM), although contemporaneity is not certain. The more hypothetical northern migration path (red) implies a pre-LGM population movement. These migration paths need not be considered mutually exclusive.

Complete agreement between mtDNA, Y-chromosomal DNA and autosomal genetic systems has not yet been realized with respect to colonization models, although all three are consistent in failing to support the ‘blitzkrieg’ or ‘three-wave’ migration models. Nevertheless, these models and their underlying assumptions continue to be used as the framework for hypothesis testing in American colonization scenarios. There is an unquestionable need for more genetic data from under-sampled geographic regions, as well as from more, and more widely dispersed, ancient populations. Because of the presumed nature of the colonization, reconstructing the genetic history of the Americas should be relatively simple compared to the challenges presented by other continents, but genetic analyses of American populations continue to be hindered by inadequate geographic (and temporal) sampling, lack of standardization of analytical methods, and the heterogeneous patchwork of diversity resulting from post-contact admixture.